For the reliable production of platelike webs of thermoplastic synthetic resin materials of a given thickness, e.g. for cutting up into deep-drawing blanks, depending upon the type of synthetic resin of thermoplastic resin or thermoplastic material involved and the speed with which the latter is extruded and fed to the rolling unit, a bulge of the material, referred to generally as a bank, can form at the upstream side of the nip or gap between the intake rolls of the rolling unit.
This bank should, in thickness, not exceed a maximum thickness or fall below a minimum thickness if defects in the finished strip of the rolled material and thus in the finished plates or on the surfaces of the resulting web are not to arise. The tolerance range between the maximum permissible and minimum permissible thicknesses or sizes of the bank is relatively small.
It has been proposed heretofore to measure the thickness of the bank by mechanical sensors or optically, for example, by a laser triangulation method.
Mechanical measurement of the bank thickness is not sufficiently accurate. The imprecision of mechanical measurement can result in detrimental defects in the web or at the surface thereof. An optical measurement is very expensive and also requires expensive circuitry for evaluating the optical measurement since following the obtaining of an optical value, conversion thereof for regulation and control of the shape imported to the web is necessary.
Reference may be had in this respect to the Japanese Patent Document JA 62 33 615 A.
In this latter system, bearing pressure on an intake roll of a rolling unit is measured and controlled and upon deviation of an actual value of the bearing pressure from a setpoint value, an alarm signal is generated so that the process can be interrupted and the system adjusted to accept a fresh feed of the thermoplastic strip.
This system cannot provide effective control of the thickness of the web as is desirable.
The bearing pressure of an intake roll will be understood to mean the pressure which the rolling of the web applies to the bearings of the roll defining the nip through which the web or strip passes.